A non-volatile memory device is a type of memory capable of retaining the stored information even in absence of electrical supply. Among the non-volatile memories presently available in the market, one of the most widespread is the flash memory.
A flash memory device is integrated in a semiconductor chip and includes one or more blocks, each one formed in a respective insulated well (body). Each block comprises a matrix of memory cells having a plurality of rows and columns; the memory cells of each row are coupled to a respective row line, while the memory cells of each column are coupled to a respective bit line. Typically, each memory cell includes a floating gate MOS transistor, insulated by means of a thin oxide layer.
An unprogrammed transistor has a reduced threshold voltage; therefore, when the transistor is selected, current flows through the respective bit line (corresponding to a logic value 1). The transistor is programmed by injecting an electrical charge into its floating gate. In this condition, the transistor has a high threshold voltage; therefore, when the transistor is selected, little or no current flows through the respective bit line (corresponding to a logic value 0).
A drawback of the flash memories is that the programmed memory cells cannot be individually erased (i.e., brought to the logic value 1), but is erased by blocks. Particularly, in an erasing operation, the body wherein such block is formed is biased in such a way to remove the electric charge included in the floating gates of the programmed memory cells.
The smallest practical block size that may be implemented at a reasonable cost amounts to few Kilobytes. In any case, the resolution obtained in this way is insufficient for those applications that require the modifying of information at the level of a single word, byte, or even bit.
Other than the standard reading, writing and erasing operations, a flash memory device may be capable of managing more complex operations. For example, U.S. Pat. No. 5,715,423, which is incorporated by reference, discloses a flash memory device capable of executing an internal copying operation, by means of which the information stored in a first group of memory cells may be copied into a second group of cells of the same memory without having to involve any external device. However, even an operation of such type may be hindered by the low resolution that is typical of flash memories; indeed, the content of the memory cells of the second group are made equal to the logic value 1 before the copying operation; otherwise the memory cells of such group would have to be first erased, which would involve all the memory cells of the block in which the second group is located.
An embodiment of the present disclosure overcomes the drawbacks previously cited.